This invention relates generally to marking of semiconductor dice bearing integrated circuits (ICs) and, more specifically, to a system for laser-marking exterior surfaces of dice carried in large groups on trays and a method of system operation.
Since the first packaged integrated circuits (ICs) became commercially available, manufacturers have often found it necessary to identify packaged ICs by marking each IC or packaged assembly of ICs with the manufacturer's name, a part or serial number, or other identifying information such as a lot number or a wafer and/or die location. As the majority of ICs are packaged individually in a transfer-molded filled polymer compound, most current marking systems have been developed for this type of IC packaging.
Manufacturers initially marked packaged ICs using mechanical ink transferring devices, such as stamps or rollers, with or without stencils, to transfer ink to the surface of an IC. One example of an ink-marking apparatus is disclosed in U.S. Pat. No. 5,226,361 to Grant et al. Because of the mechanical nature of the process and the drying time associated with ink, ink stamping systems are relatively slow and the applied ink susceptible to smudging. Also, the quality of ink-stamped marks on packaged ICs can vary substantially over time and from IC to IC due to variations in the quality and quantity of ink applied, ambient temperature and humidity, and the condition and finish of the surface of the stamp and the package.
Because of the deficiencies associated with ink stamping, manufacturers have in recent years switched to using a laser beam to mark the surface of a packaged IC. Unlike ink stamping, laser marking is very fast, requires no curing time, produces a consistently high quality mark, and can take place at any point in the manufacturing process.
Various machines and methods have been developed for marking ICs with a laser. As illustrated in U.S. Pat. No. 5,357,077 to Tsuruta, U.S. Pat. No. 4,945,204 to Nakamura et al., U.S. Pat. No. 4,638,144 to Latta, Jr., and U.S. Pat. No. 4,375,025 to Carlson, a packaged IC is placed in a position where a laser beam, such as that produced by a carbon dioxide or neodymium-yttrium-aluminum garnet laser, inscribes various characters or other information on a package surface. The laser beam burns away a small amount of material on the surface of the IC package so that the area where the characters are to appear has a different reflectivity from the rest of the package surface. By holding the packaged IC at a proper angle to a light source, the characters inscribed on the device by the laser can be read.
U.S. Pat. No. 5,937,270 to Canella (“the '270 Patent”), assigned to the assignee of the present invention and hereby incorporated herein by this reference, discloses yet another laser marking system which is operable at high throughput volumes and makes substantially constant use of a marking laser by use of a multi-track IC feed, marking and inspection procedure. While highly successful, the laser marking system of the '270 Patent feeds singulated, packaged ICs from tubular magazines along two parallel, inclined tracks to a marking zone, after which the marked devices are then automatically inspected and either discarded or re-loaded into other tubular magazines at the output ends of the tracks.
Recently developed IC packages, however, are now much-reduced in size, thickness and dimensions of individual features, such as leads for external connection to higher-level packaging. One example of such state-of-the-art IC packages is a thin plastic package configuration identified as a Thin Small Outline Package, or TSOP. Another is a Thin Quad Flat Pack, or TQFP. By way of comparison, such packages are dimensioned with a total package thickness, excluding lead fingers, of less than about one-half the thickness of a conventional plastic Small Outline J-lead package, or SOJ, such as would be marked in the above-described system of the '270 Patent. These newer IC packages, with their smaller dimensions and more fragile components, are much more susceptible to inadvertent damage in handling than prior package designs and, at best, are only marginally robust enough for handling in tubular magazines and by singulated feed-through processing equipment. As a result, the industry has gravitated to processing such relatively delicate IC packages in batches carried in recesses of rectangular trays, one example of which is so-called JEDEC trays. Other, even smaller IC packages under current development and most recently introduced to the market include so-called “chip scale” IC packages. These packages, having dimensions approximating those of a bare IC die itself and employing extremely minute external connection elements, also are desirably handled in trays. It is contemplated that such chip scale packages may be desirably laser marked on the bare, or thinly coated, backside of the die itself in instances where packaging is largely intended to protect and seal the active surface at the die sides and primarily extends over the sides and active (front) surface of the die. Accordingly, as used herein, the terms “IC package,” “packaged IC” or “IC” include not only conventional polymer-encapsulated dice but any dice incorporating sufficient structure to effect operative connection to a higher level package such as a circuit card, or to another die.
In addition to the aforementioned difficulties with marking thin, reduced-dimension IC packages using tubular magazines and inclined tracks, feeding and marking singulated IC packages, even when grouped for marking, are time-consuming and fraught with potential for workpiece jamming somewhere on the tracks. Further, such an approach requires numerous sensors to verify passage of individual IC packages, location of individual IC packages for marking and inspection, and counting of IC packages to ensure full output magazines, but not magazine overfilling and jamming of the handling equipment for same. Further, movable stops are required to locate and release the IC packages at numerous locations and so, along with the proliferation of sensors, necessitate a somewhat complex and relatively expensive control apparatus for reliable system operation.
Another disadvantage of conventional laser marking systems lies in a safety requirement that the IC packages be enclosed in a laser light-secure enclosure to prevent injury to personnel from the laser beam. Such conventional laser marking systems employ a workpiece path extending in a single plane through the marking station, thus requiring movable access shutters which must be manipulated, resulting in additional system cost and reducing throughput due to the time lost in opening and closing the shutters for entry and exit of groups of IC packages as well as adding another timed operation to the sequence of events in the marking process.
While trays facilitate moving large batches of packaged ICs while minimizing the risk of physical damage from handling, a problem with using trays to carry IC packages for marking is the need to deal with a wide range of tray-to-part tolerances. Thus, it would be necessary to orient the IC packages in the tray recesses to a common corner of each tray pocket to obtain a repeatable marking of all the IC packages in the tray. It would also be necessary to ensure that trays carrying IC packages are received in various handling and processing mechanisms in the correct orientation, so that the IC packages themselves will be properly oriented.
Accordingly, there is a need in the art for a mechanically and electrically straightforward, tray-based laser marking system which provides high, reliable throughput.